This device draws its power from whatever is connected to its MIDI in. The literature seems to indicate its draw is 5mA. My current MIDI chain gets its power from the 400mA port on a Mondo which then powers a HEX which phantom powers a GCP.

Will my current setup support adding this MIDI splitter? I plan to place it after the HEX so the entire MIDI chain would look like this..

The MIDI Solutions box is actually powered via the MIDI Data signal itself, so yes, it should be fine to run it this way.

For your Phantom Power line, I would suggest using a Current Doubler Adapter (the one with the Green end) connected to two of the 9V 400mA outputs to power HEX and the Ground Control Pro. A single output is a little on the low side for this setup.